feat: Allow adding entries to the OpenSearch keystore

[labrenbe]

Description

Allow referencing Secret keys in the OpenSearch config to add them as entries to the OpenSearch keystore.

Part of #44.

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

Release Notes

Allow adding secrets to the OpenSearch keystore, e.g. S3 credentials for creating and restoring backups.

Author

• Changes are OpenShift compatible
• CRD changes approved
• CRD documentation for all fields, following the style guide.
• Helm chart can be installed and deployed operator works
• Integration tests passed (for non trivial changes)
• Changes need to be "offline" compatible
• Links to generated (nightly) docs added
• Release note snippet added

Reviewer

• Code contains useful comments
• (Integration-)Test cases added
• Documentation added or updated. Follows the style guide.
• Changelog updated
• Cargo.toml only contains references to git tags (not specific commits or branches)

Acceptance

• Feature Tracker has been updated
• Proper release label has been added
• Links to generated (nightly) docs added
• Release note snippet added
• Add type/deprecation label & add to the deprecation schedule
• Add type/experimental label & add to the experimental features tracker

[siegfriedweber]

LGTM

[lfrancke]

Should this be mentioned in the release notes?

[labrenbe]

I've added a release notes section to the issue.

[lfrancke]

Can you explain what this would be used for? Why do we need this?